A user input mechanism that allows fast, efficient four-way control.
A trackball consists of a small sphere embedded in a device partially exposed, so that the user can move a thumb or finger against it to rotate the sphere in any direction.
Therefore, moving a thumb or finger upward while touching the trackball moves "up" in the device's user interface. The action may be represented by a cursor icon moving upward, or by scrolling up through a list.
Unlike four-way buttons (a "d-pad") a trackball allows you to scroll through many options in one motion, at nearly any speed.
Unlike a wheel or dial that only lets you scroll up and down, a trackball lets you move in four or all directions.